Recently, an electric power generator set comes to be used for supplying electric power consumption apparatuses (loads) with electric power, wherein the electric power generator set can connect a circuit of electric power (internal electric power) generated by a generator driven by a private engine to an external electric power system line typically supplied by a commercial electric power supply, such as an electric power company.
An internal combustion engine, such as a gas engine or a diesel engine, is applied as the engine for driving the generator of the electric power generator set.
The electric power generator set comprises an inverter for supplying electric power to the load so as to bring the total of electric power generated by the generator and external electric power into agreement with the electric power demanded by the load. Such an electric power supply system having a system line of electric power generated by the generator in interconnection with the external electric power system line is referred to as an interconnection system.
On the other hand, recently, a cogeneration system is widely used as an application with the electric power generator set having the above-mentioned interconnection system, which recovers waste heat generated from the generator generating electric power so as to supply hot water by using the recovered heat.
The reason why the interconnection system employs the cogeneration system having both heat and electric power produced from an engine is to efficiently and economically use fuel (gas, light oil or the like) for driving the engine. Therefore, it is preferable that the system always ensures efficiency in outputting the heat and electric power to be consumed regardless of demand status thereof.
In addition, in the light of economic efficiency (energy efficiency) of the cogeneration system, it is preferable that the rate of generated electric power in the demanded electric power (load electric power) becomes large as much as possible and demand of water-heat energy is larger than water-heat energy generated by the cogeneration system.
However, the energy efficiency of the cogeneration system changes corresponding to energy consumption status of a user, so that the user must manage operation status of the cogeneration system and collects and analyzes data for calculating energy efficiency so as to grasp the optimal conditions of operation. Such complicated collection and analysis of the data prevents the user from easily and quantitatively grasping the reduction amount of energy cost (cost merit) as the benefit of introduction of the cogeneration system.
As understood from the above example of the conventional electric power supply system employing the cogeneration system, the conventional electric power supply system combines external electric power and generated electric power in efficient rate so as to supply the combined electric power, however, it is not sufficiently user-friendly because it does not clarify its usage pattern. The electric power system would be greatly satisfying for the user if it clarified the usage pattern of external electric power and generated electric power caused by it.
A well-known conventional mode of the above-mentioned electric power system is a package type electric power system having a plurality of parallel-connected electric power generator sets each of which comprises an engine, a generator, an inverter and a control unit so that each of the electric power generator sets outputs electric power via its inverter so as to input it into the external electric power.
In the package type electric power system, the control units of the respective electric power generator sets interconnect with one another so as to control output of each electric power generator set and the number of the electric power generator sets in operation, thereby controlling output of the whole package type electric power system.
With regard to output control of the whole package type electric power system, the control unit of specific one of the electric power generator sets serves as a master unit centralizing control over the other electric power generator sets (control units). The inverter in the specific one electric power generator set detects electric current from the commercial electric power system, and the control unit as the master unit controls outputs of the control units of the other electric power generator sets on the basis of detected results.
However, in the conventional electric power system of the parallel input type, only the specific electric power generator set detects the electric current from the commercial electric power system so that each of the electric power generator sets other than the specific one cannot function as the master unit because its inverter does not detect the electric current.
Therefore, the specific electric power generator set must be constantly operated to control the output on the basis of detection of current value of the commercial electric power system, thereby causing problems, such as its operation time and frequency unevenly larger than the other electric power generator sets and short period of its consumable parts. Furthermore, at the time of maintenance of the specific electric power generator set, the other electric power generator sets must be also stopped (because the other electric power generator sets cannot control output), whereby the generated electric power cannot be supplied at the time of maintenance.